FIG. 1 illustrates a prior art switching power supply having a controller with analog voltage mode feedback control. The system of FIG. 1 includes a power path 10 that controls the flow of power from a power source to the output in response to the modulation signal MOD. The power path may embrace any suitable switching power supply topology and therefore may include any suitable number and combination of switches, transformers, inductors, capacitors, diodes, and the like. The modulation signal MOD is provided by a pulse width modulation (PWM) control arrangement in which a comparator 12 compares an error signal VERR to a ramp signal RAMP that is generated by an oscillator 14. A resistor RR sets the size of the ramp signal.
The error signal VERR is generated by an error amplifier 16 which has a feedback network including resistor RA and capacitors CA and CFB, and an input network including resistor RB and capacitor CB. The power supply output voltage VOUT is sensed by the input network, and a reference signal VREF determines the set point to which the output is regulated. The reference signal may be offset by a current feedback signal, for example, a droop voltage may be provided for active positioning of the output voltage as a function of current, commonly known as active voltage positioning (AVP).
Switching power supplies are often used for demanding devices such as high-performance microprocessors, and therefore, their controllers must be carefully tuned to respond to a wide array of steady-state and transient load conditions over their entire operating range. The design process typically begins by selecting a value for resistor RB which, in combination with current source IFB, determines the offset of the nominal power supply output voltage at no load. Next, the ramp resistor RR is selected to provide the best combination of thermal balance, stability, and transient response. Finally, the values of CA, RA, CB and CFB are selected to provide the feedback loop compensation with the best possible response to a load transient.
Equations have been developed to determine the best known stating points for all of these component values. The optimal values of the components, however, must typically be determined through a tuning process in which a circuit is first built using the calculated values of components and then subjected to testing under actual load conditions. The component values are then adjusted through a trial-and-error process to provide proper load line setting, transient response, etc.
Adjusting component values is a time consuming process. Once a seemingly optimal value of one component is determined, the value of another component must then be determined. After changing the second component, however, the first component may need readjustment. This is an undesirable burden even in a system having just one or two components to adjust. In a system having upwards of five or six different tuning components, the time required to repeatedly remove, reinstall and retest the components may become excessive.